The present invention relates to a worm wheel and an electric power steering system including the worm wheel.
A speed reduction gear is used in an automotive electric power-steering system (EPS). For example, in a column-type EPS, the rotational force of a motor is transmitted to a worm and then to a worm wheel, whereby the rotational speed of the motor is reduced so as to amplify the output of the motor to thereby assist with torque for steering operation.
Incidentally, while an appropriate backlash is necessary for mesh engagement between the worm and the worm wheel, when driven on, for example, a rough road such as a road paved with stones or bricks, a beating sound attributed to the backlash may be generated by virtue of reaction from tires, or a rattling sound may be generated from bearings and other components having plays. When these sounds are transmitted into the passenger compartment as noises, the driver is rendered uncomfortableness.
Then, to cope with this, a worm wheel is normally used which includes an annular metallic core portion and a rim portion made of a resin which is fitted on an outer circumference of the core portion and in which a toothed portion is formed on an outer circumference of the rim portion (refer to, for example, JP-A-11-301501).
In recent years, however, there exists a demand for higher outputs from electric power steering systems, and due to this, the connecting strength between the core portion and the rim portion is desired to be ensured also under a high-temperature environment.